liquid-to-liquid heat exchangers for use submerged in an automobile radiator for transferring heat from the transmission oil to the engine liquid coolant are generally constructed of three major items, a cylindrical outer tube, a cylindrical inner tube, and a turbulator constructed from formed strip sandwiched between the tubes. There are a number of disadvantages to this construction, as for example: there are three separate major items to manufacture and assemble; the additional heat transfer area provided by the turbulator strip communicates to the heat sink (engine coolant) through a mechanical bond with resulting thermal resistance, the value of which depends upon the pressure exerted between the components; the heat exchanger surface presented to the engine coolant is smooth and has no enhanced heat transfer characteristics; and finally, the increased heat transfer characteristics are gained by turbulating the entire fluid path with an attendant, and undesirable, pressure drop increase.
The aforementioned disadvantages of existing oil coolers are not inconsequential. Being able to obtain equivalent performance at a lower cost is always desirable. However, in the case of some automotive applications, the use of smaller engines and the desire for lower hood profiles has resulted in the adoption of radiators of a very small size which place absolute limits on the amount of space available for a submerged oil cooler. A typical oil cooler for a small car has a maximum length of about 111/2 inches and an outside diameter of about 1 inch. A cooler made in such a size in the conventional manner is adequate for most purposes but can prove to have insufficient oil cooling capacity for certain extreme driving conditions which place additional demands on the transmission. It would thus be desirable to have an oil cooler which could transfer more heat than conventional coolers of the same size, do so with the same or a lesser degree of pressure drop, and do so at the same or a lower cost than present coolers. A cooler disclosed in my copending application Ser. No. 574,989, filed May 27, 1975, accomplishes these goals by means of a two-piece unit having an outer tube which incorporates a plurality of longitudinal flutes which are periodically transversely indented and press fit over an inner tube having a helically finned outer surface. However, the disclosed cooler is not too readily manufacturable on available equipment and it is therefore desirable to have a cooler which can be made more easily using existing equipment.